In a conventional automatic transmission with an overdrive device, a planetary gear device is used for increasing the rotational speed of the output of the transmission. In such a construction, a pair of planetary gear devices must be provided on an axis, which further requires providing at least two transmitting devices comprising a clutch, a band and a one-way clutch. Such a construction makes the apparatus large and complicated, which also causes complications in the hydraulic control system. In a vehicle with an automatic transmission, since the gear shifting occurs automatically, it is desirable for an overdrive transmission also to be automatically performed from any transmission condition and for a return to a proper transmission condition to be made in response to vehicle speed when the overdrive condition is released by a kickdown operation.
Considering these situations, the present invention seeks to provide a control system for an automatic transmission in which the overdrive transmission is performed through the automatic transmission and an overdrive device comprises an overdrive clutch and overdrive gears.
According to the present invention, there is provided in a control system for an automatic transmission with an overdrive device for engine powered vehicles in which power from the engine is transmitted to a final reduction gear for driving the vehicle wheels through a planetary gear type automatic transmission having a forward clutch, the improvement comprising a fluid operated overdrive clutch for transmitting the power to the driving wheels of the vehicle through the overdrive device, a hydraulic circuit for supplying pressure oil to the forward clutch, an overdrive clutch provided in the hydraulic circuit, an electromagnetic overdrive valve provided in the hydraulic circuit, a manually operated overdrive switch provided in the electric circuit for said electro-magnetic valve, the electromagnetic overdrive valve being arranged such that the pressure oil is supplied to the forward clutch for the engagement thereof when the overdrive switch is not operated, and the pressure oil is supplied to the overdrive clutch for the engagement thereof upon closing of said overdrive switch for establishing the overdrive transmission.